Read the four sleeve measurements
Compare the opening, cavity depth, outside width and added tip independently.
Start with the manufacturer’s fit range
A product page may list a single unstretched diameter without stating how it translates to a comfortable fit. Measure as the manufacturer directs and compare only with the same kind of figure. Ask for a usable fit range if the listing supplies only the outside width.
Separate opening from cavity depth
An opening describes the entrance. Cavity depth describes the space inside a closed sleeve. A listed width can describe the outside. Compare those columns independently and leave any unlisted measurement open until you can confirm it.
Use an actual listing as a worked example
The selected Jock 3-inch variant lists 9 inches overall, a 6-inch hollow depth, and a 3-inch solid tip. Its opening is listed as 1.25 inches unstretched. Those are four different facts. An overall length alone cannot tell you how much is hollow or how the opening fits.